Piezoresistive pressure transducers have a wide range of applications where accurate pressure monitoring is desirable. Typical applications include process monitoring, aerodynamics, engine testing, rotating machinery monitoring and testing, nuclear power and others. Generally, such transducers include a force collector and one or more piezoresistive elements. Frequently, it is desirable that such transducers are of small physical size and capable of operating at high temperatures.
One type of pressure transducer utilizes organic epoxy to bond sensors to a metal force collector. However, such transducers have disadvantages. A disadvantage is that repeated flexing of the force collector in response to pressure changes generally causes a weakening of the epoxy bond, thus degrading the accuracy of the transducer. In addition, the sensitivity of such transducers is typically low and degrades when the transducer is subjected to temperature variations. This limits the effective temperature range over which such transducers can be utilized.
Another type of prior art pressure transducer includes silicon (Si) sensors formed integrally with a Si force collector. Such monolithic transducers have pn junctions that isolate the sensors from the force collector. This type of transducer typically permits higher temperature operation. As is well known, however, the isolation between the sensor network and the force collector in such transducers deteriorates as a function of temperature. This is caused by thermally generated carriers which short circuit the sensors to the substrate. Typically, the isolation in such transducers deteriorates at temperatures above 350.degree. F., causing a short and a degradation in performance.
One method utilized to circumvent the shorting problem associated with Si includes fabricating dielectrically isolated sensors. This is shown in U.S. Pat. No. 3,800,264 entitled HIGH TEMPERATURE TRANSDUCERS AND HOUSING INCLUDING FABRICATION METHODS which issued to A. D. Kurtz et al. and assigned to Kulite Semiconductor Products, Inc., the assignee herein. See also U.S. Pat. No. 3,930,823 issued on Jan. 6, 1976 to A. D. Kurtz et al. entitled HIGH TEMPERATURE TRANSDUCERS AND HOUSING INCLUDING FABRICATION METHODS. Transducers utilizing dielectrically isolated sensors are operable at temperatures in excess of 500.degree. C. However, above 600.degree. C. the Si sensing network and the Si force collector undergo plastic deformation, thus rendering the transducer inoperable.
Other materials have been investigated in order to provide a sensor that is operable at higher temperatures. Recent research has shown that silicon carbide (SiC) is an excellent semiconductor for high temperature applications and for use as a sensing element. Several high temperature devices have been manufactured from SiC, such as pn junction diodes that exhibit excellent rectification at 600.degree. C. This is shown in an article entitled "High Voltage 6H--SiC p-n Junction Diodes" by Matus, L. G., Powell, J. A., and Salupo, C. S. in American Institute of Physics, Applied Physics Lett., Vol. 59, No. 14, pgs. 1770-1772, on Sep. 30, 1991.
In addition, SiC has been utilized for the fabrication of high temperature pressure transducers. In this regard, reference is made to copending U.S. patent application Ser. No. 07/694,490 entitled HIGH TEMPERATURE TRANSDUCERS AND METHODS OF FABRICATING THE SAME EMPLOYING SILICON CARBIDE, filed on May 2, 1991, and assigned to Kulite Semiconductor Products, Inc. Therein a high temperature transducer is described which uses pn junction isolated SiC sensing elements on a SiC force collector of opposite conductivity type. SiC enables a transducer to be operable at higher temperatures than previous transducers.
An important operating parameter for a pressure transducer is the level of the output signal it provides. A material property that this level depends on is the gauge factor associated with the material being used. Generally, a higher gauge factor yields a higher output signal level.
However, as is well known in the art, a disadvantage in utilizing SiC is that its gauge factor is relatively low. As shown in above noted U.S. patent application Ser. No. 07/694,490, the gauge factor associated with SiC is approximately 31. As is well known in the art, gauge factors in this range will cause a transducer using SiC to produce relatively low output signals. It is desirable to provide a transducer having a significantly higher gauge factor than that of SiC so that the transducer may provide higher output signals and that is operable at temperatures above which Si transducers undergo plastic deformation.